The cowbell part is an example where one player has one instrument, so the part is notated on one line both for clarity and saving space.
The single line is specified within the attributes element for the part:
<attributes> <divisions>2</divisions> <key> <fifths>0</fifths> <mode>major</mode> </key> <time symbol="common"> <beats>4</beats> <beat-type>4</beat-type> </time> <clef> <sign>percussion</sign> </clef> <staff-details> <staff-lines>1</staff-lines> </staff-details> </attributes>
How do we determine the display-step and display-octave for a one-line staff? Since the percussion clef is treated like a treble clef, the G in octave 4 is on the second line of the staff. For this one-line staff, that G is one imaginary line above the staff (for a 2-line staff, it’s the top line, and the middle line on a 3-line staff).
Therefore the unpitched elements for the cowbell part are:
<unpitched> <display-step>E</display-step> <display-octave>4</display-octave> </unpitched>